(a) What is the difference between hybrid orbitals and molecular orbitals? (b) How many electrons can be placed into each MO of a molecule? (c) Can antibonding molecular orbitals have electrons in them?

Hybrid orbitals and molecular orbitals are two different concepts in chemistry that help explain the bonding and arrangement of atoms in molecules. Hybrid orbitals are mixtures of atomic orbitals within an atom. This mixing or hybridization happens when atomic orbitals with similar energy levels combine to form new orbitals with a hybrid character. The purpose of the hybridization is to achieve a more stable configuration for bonding with other atoms. Molecular orbitals, on the other hand, are formed by the overlap of atomic orbitals from different atoms. They are mathematical functions used to describe the spatial distribution and behavior of electrons in a molecule. When atomic orbitals overlap, they form molecular orbitals that can be either bonding or antibonding, depending on the type of overlap.

Each molecular orbital (MO) can hold a maximum of two electrons, following the Pauli Exclusion Principle. The principle states that no two electrons in a molecule can have the same set of quantum numbers. Therefore each molecular orbital can accommodate two electrons having opposite spins.

Yes, antibonding molecular orbitals can have electrons in them. Antibonding molecular orbitals have a higher energy state than the bonding molecular orbitals and are formed due to the out-of-phase or destructive combination of atomic orbitals from different atoms. When electrons populate the antibonding molecular orbitals, they weaken the bond between the atoms and increase its energy. If the number of electrons in bonding and antibonding orbitals is equal, the bond order becomes zero, and no bond is formed between the atoms. Generally, electrons will first fill up the lower-energy bonding orbitals before occupying antibonding molecular orbitals.